1. Field of the Invention
The present invention relates generally to piezoelectric-resonance devices of an energy-trapped type utilizing a higher harmonic in the mode of thickness-extensional vibration, and more particularly, to piezoelectric-resonance devices having an improved electrode structure for energy trapping.
2. Description of Related Art
The described U.S. patent application Ser. No. 211,777 discloses a piezoelectric-resonance device which can be used in a high frequency region by utilizing a higher harmonic in the thickness-extensional vibration mode. In this piezoelectric-resonance device, a plurality of electrodes for energy trapping are arranged so as to be overlapped with each other through piezoelectric ceramic layers in a sintered body made of cofired type piezoelectric ceramics. Consequently, the piezoelectric-resonance device is constructed such that the higher harmonic in the thickness-extensional vibration mode is trapped in a region where the electrodes are overlapped with each other.
In the above described piezoelectric-resonance device, the higher harmonic in the thickness-extensional vibration mode is trapped. Accordingly, a first response in the thickness-extensional vibration mode appears in a higher frequency region, as compared with a conventional piezoelectric-resonance device in which electrodes for energy trapping are formed on both major surfaces of a single plate made of piezoelectric ceramics. Consequently, a piezoelectric-resonance device which can be used in a high frequency region can be achieved. In addition, the higher harmonic in the thickness-extensional vibration mode can be trapped using a piezoelectric material of composition having an effective Poisson's ratio of less than 1/3 which could not be previously been used in a resonance device of an energy-trapped type. Accordingly, various piezoelectric materials can be suitably selected.
In the piezoelectric-resonance device disclosed in the above described related art, however, the plurality of electrodes for energy trapping arranged in the sintered body must be formed so as to be precisely overlapped with each other through piezoelectric ceramic layers; otherwise piezoelectric characteristics would greatly vary. However, it is difficult to prevent a shift in position between the plurality of electrodes. Accordingly, there results the need for complicated work of determining characteristics of piezoelectric-resonance devices produced in large quantity and selecting acceptable ones depending on their characteristics.
On the other hand, it is considered that a structure shown in FIG. 1 is employed so as to solve the above described variation in characteristics. In FIG. 1, ceramic green sheets 1 and 2 are laminated. Conductive pastes 3 and 4 for forming an electrode for energy trapping and a connecting conductive portion are applied to the upper surface of the ceramic green sheet 1. In this specification, the same reference numerals as those of an electrode for energy trapping and a connecting conductive portion which are finally completed by sintering are assigned to portions coated with conductive pastes for forming an electrode for energy trapping and a connecting conductive portion.
A conductive paste 5a is applied on the entire upper surface of the ceramic green sheet 2. On the other hand, conductive pastes 6 and 7 are applied to portions to be an electrode and a connecting conductive portion 2.
The above described ceramic green sheets 1 and 2 are laminated and, and then are subjected to polarization processing, thereby allowing a piezoelectric-resonance device 8 shown in FIG. 2 to be obtained. In this piezoelectric-resonance device 8, an electrode 5a in the position of its middle height is formed on the whole surface of the ceramic green sheet 2. Consequently, there occurs no shift in overlapping between the electrode 5a and the electrodes 3 and 6 on its upper and lower sides.
However, stray capacitance is produced between the whole surface electrode 5a and the connecting conductive portions 4 and 7 on its upper and lower sides. This stray capacitance causes a spurious mode to be excited in the band of frequencies between a resonance frequency and an antiresonance frequency. In addition, this stray capacitance causes the antiresonance frequency and antiresonance resistance to be decreased and causes the band of frequencies between the resonance and antiresonance frequencies to be narrowed. If the piezoelectric-resonance device 8 is applied to an oscillator or the like, the stray capacitance causes failures, such as interruption of oscillation.